Heterogeneous networks (HetNets or HTNs) are now being developed wherein cells of smaller size are embedded within the coverage area of larger macro cells and the small cells could even share the same carrier frequency with the umbrella macro cell, primarily to provide increased capacity in targeted areas of data traffic concentration. Such heterogeneous networks try to exploit the spatial distribution of users (and traffic) to efficiently increase the overall capacity of the wireless network. Those smaller-sized cells are typically referred to as pico cells or femto cells, and for purposes of the description herein will be collectively referred to as small cells. Such deployments present some specific interference scenarios for which enhanced inter-cell interference coordination (eICIC) techniques would prove beneficial.
In one scenario, the small cells are pico cells, which are open to users of the macro cellular network. In order to ensure that such pico cells carry a useful share of the total traffic load, user equipments (UEs) may be programmed to associate preferentially with the pico cells rather than the macro cells, for example by biasing the received signal power of the Common Reference Symbol (CRS), a quantity that may be referred to as reference signal received power (RSRP), such that UEs that are close to a pico cell will associate with the pico cell. Despite the association, UEs near the edge of a pico cell's coverage area will suffer strong interference from one or more macro cells. In order to alleviate such interference, some subframes may be configured as “almost blank” in the macro cell. An “almost blank” subframe is a subframe with reduced transmit power (e.g., reduced from a maximum transmit power) and/or a reduced activity subframe (e.g., contains only control information as compared to a fully loaded subframe). Legacy UEs (also called terminals) expect to find the reference signals for measurements but are unaware of the configuration of these special subframes. Almost blank subframes may contain synchronization signals, broadcast control information and/or paging signals.
In order to make use of almost blank subframes (ABSs) effective (note that hereafter the term “special” or “ABS” is used), signaling is provided from the macro cell to the pico cell across the corresponding backhaul interface, known in LTE as the “X2” interface. For LTE Release 10, it has been agreed that this X2 signaling will take the form of a coordination bitmap to indicate the ABS pattern (for example with each bit corresponding to one subframe in a series of subframes, with the value of the bit indicating whether the subframe is an ABS or not). Such signaling can help the pico cell to schedule data transmissions in the pico cell appropriately to avoid interference (e.g. by scheduling transmissions to UEs near the edge of the pico cell during ABSs), and to signal to the UEs the subframes which should have low macro cellular interference and should therefore be used for RRM/RLM/CQI measurements. (RRM=Radio Resource Management, typically relating to handover; RLM=Radio Link Monitoring, typically relating to detection of serving radio link failure; CQI=Channel Quality Information, derived from the signal strength from the serving cell and the interference from other cells, and typically used for link adaptation and scheduling on the serving radio link).
EICIC is an Interference Mitigation technique that involves the transmission of ABS from a macro cluster. During the transmission of ABS, only a subset of the broadcast channels is transmitted while PDSCH is muted. This allows underlaid small cells such as metro cells, femto cells and relays to transmit to the UEs that have selected those nodes with a better SINR.
Since LTE is a co-channel deployment (i.e., it has 1:1 frequency re-use in the different cells). The edge users' uplink performance can be severely impaired due to interference received from neighboring cells that use the same frequency due to 1:1 re-use. To mitigate the neighboring cell interference that limits the performance of edge users, the standards body has proposed the following approach: periodically, each cell sets cell-specific parameters that the associated UEs of the cell use to set their SINR target as a pre-defined function of these parameters and local path loss measurements. Precisely, the standards body has proposed a Fractional Power Control (FPC)-α scheme where a UE sets its transmits power (in dBm) according to the following relationshipTx Power of UE=P0(server cell)+α(server cell)*Path_Loss(between UE and server cell)  (1)where, parameters P0 is the cell specific nominal transmit power and α is the cell-specific path loss compensation factor, both depending on the server cell of the UE. In equation (1) the transmitting power of the UE is understood to express transmitting power per Resource Block (RB).